1. Field of the Invention
The present invention relates to a novel photosensitive composition suitably used to manufacture semiconductor devices or multilayer circuit boards in the field of electric and electronics. Particularly, this invention relates to a photosensitive composition applicable to a lithography process with a short wavelength light source in order to obtain polyimide patterns after annealing. Such a composition is becoming popular in the production process of semiconductors.
2. Prior Art
Polyimides have been recognized as a material used in the field of electronics for their high thermal and chemical stabilities, low dielectric constant and excellent ability in planarization. These polyimides have been widely used as materials for surface protective layers, interlayer dielectrics of semiconductors and insulating layers in multichip modules.
In order to obtain a desired pattern from ordinary polyimide coatings, an indirect lithographic method wherein, for example, a polyimide layer with patterned photoresist as an etching mask is etched is employed. In this method, however, the process is complicated. Moreover, the method has drawbacks such as the use of harmful agents like hydrazine is required in the etching process and the resolution of the polyimide pattern is reduced because of the indirect lithographic method.
Accordingly, investigations into methods for direct pattern formation of polyimides with photoreactive compounds having polyimides or polyimide precursors have been conducted. In such methods, the polyamic acid derivatives having double bonds linked through ester bonds, amide bonds, acid ammonium salt and the like are used. The polyamic acid derivatives can be used in the direct photolithography process with photo initiators. The photosensitive components are removed by heating to obtain thermally stable polyimides (T. Yamaoka and T. Omote, "Polyfile", vol.27, no.2, pp.14-18 (1990)). This technology is generally referred to as a photosensitive polyimide technology.
A demand for accumulation of semiconductors such as IC, LSI, VLSI has been gradually increasing. According to this demand, processing technologies for fine materials has been expected. In one of the technologies, the pattern formation of photoresists is conducted by using short wavelength light such as i-line light of Hg lamp (365 nm wavelength) instead of ordinary G-line light (436 nm wavelength), for which a high resolution can be expected. It has been anticipated that all exposure apparatus will employ i-line light at semiconductor munufacturing factories in the near future.
On the other hand, in the case of conventional polyimide precursor technology, the compositions to be used in this technology have relatively high absorbance to i-line light. Further, the polyimide precursor compositions are applied to be rather thick, that is, 12 .mu.m or more, in view of the shrinkage caused by removing the photosensitive components during curing, because the polyimide coating requires 6 .mu.m thick or more from the standpoint of the physical properties. Therefore, conventional polyimide precursor compositions do not allow i-line light to reach the bottom of its coating. As a result, the bottom of the coating is not sufficiently hardened by exposure and the pattern is washed away in development. Consequently, i-line light is not suitable for the exposure light source for obtaining a polyimide coating having more than a certain thickness. (C. Schuoket, et al., "IEEE/SEMI Advanced Semiconductor Manufacturing Conference", pp.72-74 (1990)).
When i-line light is used, a pattern formation of thin coatings must be conducted several times to obtain sufficiently thick polyimide coating, so that the accuracy of the resultant patterns cannot be improved and the production process becomes complicated. Therefore, i-line light exposure is not practical.
It has been known that a film of a fluorine-containing polyimide precursor has high i-line light trasmittancy (T. Omote, T. Yamaoka and K. Kosei, "Journal of Applied Polymer Sience", vol.38, pp.389-402 (1989)). Polyimides, which are obtained by heat-curing the precursors and contain fluorine in their main frame work, have drawbacks in practical use. That is, they have a low adhesive strength to other materials used with polyimides, such as base inorganic materials, epoxy resins in coating layers and metals for distributing wires when they are used for semiconductor devices and multilayer circuit boards. A photosensitive polyimide precursor composition, which can form patterns by means of i-line light and is applicable to practical uses, has not yet been obtained.